1. Field of the Invention
The present invention is directed to a shim device for a magnetic resonance apparatus of the type having a cavity for the acceptance of shim elements.
2. Description of the Prior Art
German OS 197 22 211 discloses a shim device of the type described above. The shim device is arranged in a gradient coil system and has pocket-like cavities. The cavities are provided for accepting shim elements with which the static basic magnetic field of the magnetic resonance apparatus can be shaped and homogenized.
When imaging with magnetic resonance (MR) one prerequisite for generating artifact-free images is a high homogeneity of the static basic magnetic field in the imaging region. Minute inhomogeneities caused by fabrication tolerances and by ferromagnetic articles that are present in the region in which the apparatus is installed, can be eliminated with an arrangement referred to as a shim. To this end, the magnetic field is measured with an MR probe at a number of incident points that are uniformly distributed on a surface of an approximately spherical homogeneity volume. The field values are entered into a calculating program that calculates a suitable arrangement of iron plates to be attached in the interior of the magnet. After the assembly, a monitoring measurement is also implemented. This procedure usually must be repeated one or two times before a satisfactory shim result, i.e. an adequately highly uniform magnetic field, is achieved in the homogeneity volume and, thus, in the imaging region.
It is also possible to homogenize the magnetic field with correction coils. Because such coils are constructed in a rather complicated fashion for location-dependencies of a higher order, this technique is currently usually restricted to corrections of lower-order for cost reasons. A multi-channel power pack device is required for the operation of the shim coils that supplies extremely constant DC currents that can be reproducibly set. An electrical shim is usually employed for fine correction when extremely high uniformity is a matter of concern.
U.S. Pat. No. 4,439,733 discloses an antenna for a diagnostic magnetic resonance apparatus. The antenna has a number of conductor elements that are arranged on a cylindrical generated surface parallel to their axis. The conductor elements are arranged uniformly spaced from one another in circumferential direction. The antenna is part of a diagnostic magnetic resonance apparatus and is fashioned such that it can be introduced into a cylindrical interior of a superconducting magnet. The interior of the antenna is in turn large enough in order to accept a patient for producing magnetic resonance images.
In order to enhance the efficiency of shim measures, it is desirable that the shim elements required for the homogenization be arranged as close as possible to the examination space. At the same time, the shim elements should not disturb other function units of the magnetic resonance apparatus insofar as possible.
An object of the present invention is based on the object of specifying a shim device for a magnetic resonance apparatus that manages with little outlay for the shim elements.
This object is achieved in accordance with the invention in a shim arrangement wherein the cavity is arranged in an antenna conductor.
The shim device is thus as close as possible to the examination space, so that a high efficiency of the individual shim elements is achieved. It is also advantageous that the electrically conductive structures of the antenna conductor that surround the shim elements has an attenuating and shielding effect on electromagnetic disturbances proceeding from the shim elements. To that end, the skin effect, the shielding effect of the conductor itself, and the cut-off waveguide effect that is present at every waveguide structure can be utilized. This effect, however, causes an attenuation of the disturbances starting at the cut-off frequency because higher frequencies than the cut-off frequency can no longer be transmitted within the waveguide structure. Further, it is advantageous that changes of the effectiveness of the shim due to temperature, as can occur given the employment of ferromagnetic shim elements, are slight. This is particularly true compared to an arrangement of the shim elements within the gradient system. The reason for this is that the allowable temperature ranges in the transmission coil are more narrowly limited because of the proximity to the patient than the temperature ranges allowed given a gradient system. Finally, the antenna conductors are easily accessible, so that no significant disassembly and assembly outlay is necessary in the case of subsequently required shim measures in order to modify the distribution of shim elements.
An especially good shielding effect against electromagnetic disturbances proceeding from the shim elements themselves is established in an embodiment wherein the cavity is located completely within the conductor.
In another embodiment, the cavity extends in a longitudinal direction of the conductor, resulting in free distribution of the shim elements in the longitudinal direction.
In a further embodiment, the shim device has a container of non-magnetic material wherein the shaped parts are arranged and held in position. Despite the not inconsiderable magnetic field forces on the shim elements, the shim device thus remains easy to handle overall. In particular, the container with the shim elements can be removed from the conductor for changing the arrangement of the shim elements.